1. Field of the Invention
The present invention relates to an electronic apparatus including a printed wiring board provided with heat generating components such as a power supply circuit and a CPU, and in particular a structure for radiating the heat of the heat generating components.
2. Description of the Related Art
For example, a notebook computer incorporate a CPU. The CPU and a power supply circuit for supplying power to the CPU are mounted on a printed wiring board. The power supply circuit includes a number of circuit components located in the vicinity of the CPU. The amount of heat generated by the CPU increases as the processing speed of the CPU or the number of functions thereof increases. Also, the amount of heat generated by the power supply circuit increases with an increase in the power consumption of the CPU. When the temperature of the CPU or the power supply circuit becomes too high, the CPU or circuit components cannot operate effectively. In order to solve this problem, in conventional apparatuses, a heat sink is attached to the CPU, or circuit components are separated from each other so as not to have a thermal influence on each other.
U.S. Pat. No. 5,581,443 discloses a portable computer in which a heat sink for improving the heat radiation function of the CPU is provided in a housing. The CPU includes an IC chip which generates heat, and is mounted on an upper surface of a printed wiring board. The printed wiring board has a multi-layer structure in which a number of insulating layers and conductive layers are alternately stacked on one another. Also, the printed wiring board has a through hole formed in association with the position of the IC chip. The through hole is larger than the IC chip, and extends through the printed wiring board in the thickness direction thereof.
The heat sink is fixed to the printed wiring board, and comprises a main body and a cover. The main body is located on a lower surface of the printed wiring board. The cover is located on the upper surface of the printed wiring board. To be more specific, the main body has a convex portion at its center portion. The convex portion extends through the through hole, and its distal end surface is in thermal contact with the IC chip. The cover is in thermal contact with the IC chip through a heat-transmitting sheet. The main body and the cover are fixed to the printed wiring board by screws, as a result of which the IC chip is held between the main body and the cover.
When heat is generated from the IC chip, it is transmitted to the main body and cover of the heat sink. Thus, the heat of the IC chip is radiated into the housing through the main body and the cover.
In the above U.S. patent, the through hole through which the concave portion extends must be formed in the printed wiring board, in order that the main body of the heat sink be in thermal contact with the IC chip. Therefore, a specific step of forming the through hole in the printed wiring board must be added, thus increasing the number of steps for manufacturing the printed wiring board. Inevitably, the manufacturing cost of the printed wiring board is increased.
Furthermore, in the printed wiring board having a multi-layer structure in which insulating layers and conductive layers are alternately stacked on one another, the conductive layers must be formed to bypass the through hole. Inevitably, the wiring lengths of the conductive layers are increased. This is not desirable, especially for a high-speed circuit in which signals are transmitted at high speed.